1. Field of the Invention
The present invention relates to an image reading unit for reading out an image on a sheet material (document) comprising a document feeding unit for automatically feeding the sheet material (document) with the image recorded, used for a scanner, a copying machine, a printer, a facsimile equipment, or the like, and an image forming apparatus comprising the same.
2. Description of the Related Art
Conventionally, there is an image reading unit comprising a document feeding unit (hereinafter, it is referred to as the ADF (auto document feeder)) such as a double side compatible automatic document feeding unit used for an image forming apparatus such as a digital copying machine. According to the image reading unit used for the digital copying machine comprising an ADF of this kind, in the case where of copying a document in the ADF, reading means is moved to a set position provided below the ADF by a motor, that is, to a position substantially immediately below a platen roller in the ADF. Then, a light beam is outputted form a lamp unit so that a document is scanned while feeding the document between the platen roller and a glass plate and the reflected light beam is detected by reading means. Then, by processing in a printer part based on the detected data, a copied image can be obtained.
As shown in FIG. 12, the document reading part of the ADF of the conventional digital copying machine comprises a platen roller 524 and a document glass 516 as a transparent document guide such that a document P is fed while sliding on the surface of the document glass 516.
Moreover, in order to ensure the feeding property of the document P, there is one for preventing charging of the document glass 516 by providing an earth spring (not shown) in the document glass 516 longitudinal direction (document width direction) end part as shown in, for example, FIG. 1 of the official gazette of the Japanese Patent Application Laid Open (JP-A) No. 2001-83749.
In the conventional example, a problem is involved in that dusts such as toner particles, paper dusts, and rubber dusts occurred by the document feeding operation are stagnated on the document glass. In particular, in the case where the dusts are stagnated at the document reading position, a problem arises in that a stripe image is occurred at the time of recording an image.
That is, as shown in FIG. 12, the dusts conveyed with the document P are slid between the document P and the document glass 51. Thereafter, although the document P is detached from the document glass 516 so as to be fed along the feeding guide 517, the dusts G are attached electrostatically on the document glass 516 so as to be stagnated. At the time, if the dusts G happen to be at the document reading position, they are read out continuously by the reading means during the reading operation so as to occur the image stripes.
Moreover, since the conventional earth spring is provided in the document glass 516 longitudinal direction (document width direction) end part so that the distance between the static electricity generating area and the earth spring is long and the total resistance value is large, a sufficient antistatic effect cannot be obtained, and thus electrostatic attachment of the dusts G onto the document glass 516 cannot be prevented.
Moreover, according to the conventional earth spring, although charge of the document glass 516 can be eliminated, the charge of the document charged by the friction cannot be eliminated.